|Publication number||US6987908 B2|
|Application number||US 10/167,915|
|Publication date||Jan 17, 2006|
|Filing date||Jun 11, 2002|
|Priority date||Aug 24, 2001|
|Also published as||US6844212, US20030039442, US20040208446|
|Publication number||10167915, 167915, US 6987908 B2, US 6987908B2, US-B2-6987908, US6987908 B2, US6987908B2|
|Inventors||Aaron Bond, Ram Jambunathan, Newton C. Frateschi|
|Original Assignee||T-Networks, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Referenced by (9), Classifications (13), Legal Events (17)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims the benefit of U.S. Provisional Application No. 60/314,951, filed Aug. 24, 2001, the contents of which are incorporated herein by reference.
This invention relates to grating dispersion compensators. More particularly this invention relates to dispersion compensation gratings integrally formed within planar waveguide apparatuses.
Optical communication systems use pulses of light to transmit data. The light used to create these pulses necessarily contains a band of wavelengths, rather than a single wavelength, even though the light is generally from laser sources. A broadening of the wavelength band occurs when the light is pulsed, due to conditions imposed by the Fourier transform spectrum of the pulse shape. Some devices, such as electro-absorption modulators (EAM's), used to generate the pulses broaden the wavelength band of the light even further than the transform limit. The desire for ever higher speed optical communications requires ever shorter pulses of light, with ever larger wavelength bands. A problem related to this growing wavelength band of optical pulses is temporal broadening due to chromatic dispersion.
Many components of optical communications system, such as the optical fibers, EAM's, semiconductor optical amplifiers (SOA's), and variable optical attenuators (VOA's), have chromatic dispersion. The existence of chromatic dispersion in the system means that different wavelengths of light travel at different speeds within the system. This phenomenon may lead to temporal pulse broadening. For example, in SMF28 fiber, operating at 1550 nm, longer wavelengths of light travel more slowly within the optical fiber than shorter wavelengths. The dispersion factor, D, in this example is 17 ps/nm/km. Therefore, the shorter wavelengths in a pulse move ahead of the longer wavelengths, broadening the pulse. As the length of the fiber increases this broadening accumulates and chromatic dispersion ultimately limits the maximum distance a data stream can be transmitted before the pulses become indistinguishable due to broadening. For a transform-limited pulse operating at 40GB/S, the distance before the signal deteriorates significantly due to chromatic dispersion is approximately 4 km.
It is, therefore, desirable to compensate for this chromatic dispersion effect by introducing dispersion compensators, which have an opposite effect. A number of methods for introducing this dispersion compensation have been used, such as dispersion compensating fibers (DCF's) or fiber Bragg gratings. Generally, these methods are not easily tunable and, therefore, can only be used for compensating fixed dispersion values. It is impractical to design a specific DCF or fiber Bragg grating for every possible chromatic dispersion due to various lengths of optical fiber or other optical components in a communications signal. Also, in wavelength division multiplex systems, owing to differences in the slope of dispersion compensated fibers and standard fibers such as SMF28 fibers, each channel would need to be fine tuned individually.
In a reconfigurable optical communications network the chromatic dispersion may be varied when the path length changes due to cable damage, overload, or other rerouting. This also may be a problem for fixed dispersion compensation devices such as DCF's or fiber Bragg gratings.
U.S. Pat. No. 6,363,187 B1 to Fells et al. describes an optical waveguide provided with a linearly chirped Bragg reflective grating which can be used to provide linear dispersion compensation. The amount of dispersion compensation provided by this device can be adjusted by changing the magnitude of the axial strain imposed on the grating. Fells et al. disclose that, adjusting the linear dispersion of the grating in this manner requires the presence of a quadratic chirp term, either within the grating itself or within the strain placed upon the grating. This quadratic chirp term leads to additional difficulties and, therefore, should be compensated, at least in part, by causing the optical signal to be reflected by a second Bragg reflective grating with a quadratic component of chirp having an opposite sign to that of the original Bragg reflective grating.
Fells et al. also note that similar effects may be achieved by adjusting the effective refractive index of the waveguide grating structure by changing the temperature of the waveguide, possibly creating a temperature gradient along the longitudinal access of the waveguide in the process. In this temperature tunable dispersion compensating device disclosed by Fells et al., the temperature in both the original chirped Bragg reflective grating and the secondary Bragg reflective grating used for quadratic chirp term compensation should be varied simultaneously.
There is therefore a useful role for an adjustable amplitude linear dispersion compensation device. Such a device could be one designed for operation on its own to achieve substantially complete dispersion compensation. Alternatively, it could be one designed for operation in association with a fixed amplitude dispersion compensation device, such as a length of DCF, that provides a level of compensation which is inadequately matched on its own. The adjustable device may be operated with some form of feedback control loop to provide active compensation that can respond to dynamic changes of dispersion within the system, and in suitable circumstances to step changes resulting from rerouting occasioned, for instance, by a partial failure of the system such as a transmission fiber break.
One embodiment of the present innovation is an exemplary grating dispersion compensator (GDC), which includes a substrate, a dielectric grating layer, a planar waveguide, and a passivation layer. The dielectric grating layer is formed on the top surface of the substrate and includes a variation in index of refraction. This variation in index of refraction defines a grating period. The grating period may vary along the longitudinal axis of the GDC according to a predetermined function. The variable (chirped) grating period causes different wavelengths of light to have different transit times within the GDC. A selected center wavelength and dispersion curve may be created. The planar waveguide is formed next on the dielectric grating layer and includes an input/output (I/O) surface. The longitudinal axis is normal to the I/O surface of the planar waveguide. The passivation layer is formed on the planar waveguide. An alternative exemplary GDC may be formed in which the dielectric grating layer is formed on top of the planar waveguide rather than beneath it.
In an alternative embodiments of the present invention, an effective grating period chirp may be formed by means other than varying the actual grating period of the dielectric grating layer. These alternative embodiments allow an effective variation in the grating period whether the actual grating period of the dielectric grating layer varies or not.
One such alternative embodiment is to curve the planar waveguide. The grating period along the, curved, longitudinal axis of the curved planar waveguide may then be varied as desired due to the variation in angle between the axis of the grating and the longitudinal axis of the planar waveguide.
Another such alternative embodiment uses a variation in the index of refraction along the longitudinal axis of the planar waveguide to create an effective grating period chirp. This exemplary planar waveguide may include a quantum well structure. The thickness of the waveguide may be varied along the longitudinal axis as desired using selective area growth. This thickness variation changes the index of refraction of the waveguide due to the quantum well structure.
Other embodiments of the present innovation are exemplary dynamically tunable GDC'S, including electrically-controlled GDC's, thermally-controlled GDC's, and pressure-controlled GDC's. These exemplary dynamically tunable GDC's include a substrate, a dielectric grating layer, a planar waveguide, and a passivation layer. The dielectric grating layer includes a variation in index of refraction, defining a grating period. The grating period may be constant, or may vary along the longitudinal axis of the GDC according to a predetermined function.
An exemplary electrically-controlled GDC includes at least one substrate electrode coupled to the substrate and at least one top electrode coupled to the passivation layer. By controlling the voltages and/or currents between these electrodes, the center wavelength and/or the shape of the dispersion curve of the GDC may be tuned.
An exemplary thermally-controlled GDC includes at least one heating element coupled to the front portion of the passivation layer, near the I/O surface of the waveguide. By controlling the temperature of the heating element(s) to create temperature gradients in the GDC, the center wavelength and the shape of the dispersion curve of the GDC may be tuned. Exemplary heating elements may be resistive heating elements or a thermoelectric coolers. In one alternative embodiment a resistive heating element is formed over the passivation layer such that the linear resistivity, and therefore the heat generated, varies along the longitudinal axis. A first electrical contact is coupled to the front portion of the resistive heating element, near the I/O surface of the planar waveguide, and a second electrical contact is coupled to the back portion of the resistive heating element, near the other end of the planar waveguide.
An exemplary pressure-controlled GDC includes a plurality of electrodes coupled to the sides of the passivation layer and at least a portion of the passivation layer includes a piezoelectric material electrically coupled to the first electrode and the second electrode. By controlling the internal pressure of created by the piezoelectric material in the GDC, the center wavelength and the shape of the dispersion curve of the GDC may be tuned.
A further embodiment of the present invention is an exemplary apodized GDC. Exemplary apodized GDC's are formed by applying an apodization technique to a previously described exemplary GDC. Exemplary apodization techniques include: forming a spacing layer with a variable thickness between the dielectric grating layer and the waveguide; and varying a dimension, either thickness or width, of the dielectric grating layer along the longitudinal axis of the waveguide.
Yet another alternative embodiment of the present invention is an exemplary dispersion compensated optical signal detection apparatus. The exemplary dispersion compensated optical signal detection apparatus includes a circulator, an exemplary GDC of the present invention, and an optical detector. The exemplary GDC and the optical detector are optically coupled to the circulator.
A further alternative embodiment of the present invention is an exemplary dispersion compensated optical signal amplification apparatus. This dispersion compensated optical signal amplification apparatus includes a first optical fiber, a circulator, an exemplary GDC of the present invention, a semiconductor optical amplifier (SOA), and a second optical fiber optically coupled to the SOA. The first optical fiber, exemplary GDC, and SOA are optically coupled to the circulator. The second optical fiber is optically coupled to the SOA.
In an alternative exemplary embodiment of the dispersion compensated optical signal amplification apparatus, the SOA is between the circulator and the exemplary GDC and the second optical fiber is optically coupled to the circulator instead of the SOA.
Still another alternative embodiment of the present invention is an exemplary monolithic optical chip. An exemplary monolithic optical chip includes a substrate, a waveguide layer, and a passivation layer. In the back portion of the exemplary monolithic optical chip, the substrate includes a second layer, which forms an optical grating section, and the waveguide layer includes a GDC waveguide section. These sections form a GDC. In the front portion of the exemplary monolithic optical chip a second optical device is formed within the monolithic structure. This second optical device may be an active optical device, or a passive optical device. For an exemplary embodiment including an active second optical device, the waveguide layer includes an active layer section of planar waveguide and an I/O surface at the front end of the monolithic optical chip. A first electrode is coupled to the substrate and a second electrode coupled to the passivation layer over the active device section of the waveguide layer. For an exemplary embodiment including the passive second optical device, the waveguide layer includes an input surface, an output surface, and a circulator section within the front portion of the monolithic optical chip.
Additional alternative embodiments of the present invention, include exemplary methods of manufacturing exemplary GDC's. A first exemplary method of manufacture includes several steps. A substrate is provided. A grating layer base is formed on the substrate, then defined and etched to form a grating pattern. A grating top layer is formed over the etched grating base layer such that the grating top layer and the grating base layer form a dielectric grating layer having an index of refraction that changes to define at least one grating period. A waveguide layer is formed over the dielectric grating layer and defined and etched to form a waveguide. A passivation layer is formed over the waveguide. A waveguide input/output surface is then formed on the waveguide, often by cleaving the GDC from a wafer.
A second exemplary method of manufacture includes many of the same steps. A substrate is provided. A waveguide layer is formed over the substrate and defined and etched to form a waveguide. A spacing layer is formed on the waveguide and the substrate. The spacing layer includes a grating layer base, which is defined and etched to form a grating pattern. A grating top layer is formed over the etched grating base layer such that the grating top layer and the grating base layer form a dielectric grating layer having an index of refraction that changes to define at least one grating period. A passivation layer is formed over the grating top layer. A waveguide input/output surface is then formed on the waveguide, often by cleaving the GDC from a wafer.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
One exemplary embodiment of the present invention, illustrated in
Passivation layer 108 and chirped optical grating 104, and preferably substrate 102 as well, have a lower index of refraction than waveguide 106 and act as cladding layers. The materials for these three layers may desirably be selected from a related family of materials to the material used to form waveguide layer 106. Such a material selection may minimize scattering at the boundaries between layers and may also improve the quality of crystalline materials by reducing lattice mismatches between layers.
Additionally, waveguide 106 may contain a number of sub-layers, forming a quantum well structure within this layer. A quantum well waveguide structure may be desirable to increase the tunability of the index of refraction within the waveguide. This structure may include a single quantum well, multiple quantum wells, or separate confinement layers. Substrate layer 102 and passivation layer 108 may also contain a plurality of sub-layers. It is often desirable for passivation layer 108 to extend around the waveguide on the sides as well as over the top of waveguide 106, as shown in
In addition to serving as a cladding layer, passivation layer 108 may also desirably function as the p-type material of a P-I-N structure. A quantum well structure may be particularly desirable for exemplary tunable GDC's, such as those described below with respect to
As shown, it is generally desirable for chirp of optical grating 106 to progress from larger spacings in the index of refraction variation, known as the grating period, near input/output (I/O) surface 107 of waveguide 106, to narrower spacings along the longitudinal axis of device 100, i.e. the propagation axis of waveguide 106. This arrangement leads longer wavelengths to be reflected closer to I/O surface 107 and therefore to have a short transit time within exemplary GDC 100. This grating period chirp may be linear, or it may be determined by a predetermined function to more accurately match the chromatic dispersion to be compensated.
Even though the majority of the light is confined within the waveguide, proper selection of indices of refraction in the device may desirably allow enough of the optical field to extend into the chirped optical grating for Bragg reflection to occur at resonant wavelengths, similar to methods employed to create feedback in distributed feedback (DFB) lasers. Resonant wavelengths are those wavelengths where an integral number of wavelengths are substantially equal to twice the grating period. The grating period is based on the effective optical distance between the alternating material portions of the grating element. The period of chirped optical grating 104 is varied along the longitudinal axis of the device. Therefore, different wavelengths are reflected at different distances due to diffraction from chirped optical grating 104. Desirably, larger grating periods occur near the input of the device and the first order Bragg reflection is significant. Such a chirped optical grating design may help to limit losses due to secondary reflections back into the device, as well as substantially limiting reflections to first-order Bragg reflections only. Apodization methods, described below with respect to
Alternatively, it is contemplated that a negative chirp may be created in the chirped optical grating, with shorter grating periods near the input of the device. Such a negatively chirped GDC may be used to fine tune over-compensated systems, or may be tailored to other applications for which negative dispersion compensation may be desirable. The relatively narrow bandwidth most pulsed laser systems desirably reduces the effect of second-order, or higher, Bragg reflections.
It is not always practical to couple light emitted from optical fibers into a planar waveguide as either a pure TE or TM mode. Therefore it is desirable to account for both TE and TM modes during design. Inefficiencies may occur in exemplary GDC 100 due to possible dispersion between TE and TM modes in waveguide 106. It is, therefore, desirable to limit this polarization modal dispersion. Designing waveguide 106 to have an approximately square cross-section, as shown in
The passivation layer is also shown as two sub-elements, side passivation layers 304 and top passivation layer 306. It may be desirable for these passivation layer sub-elements to have different compositions in exemplary tunable GDC's described below, particularly those shown based on electrical tuning (
Exemplary GDC 100 may be designed for 1.55 μm optical signals, using III/V materials. Such a device may be formed on an InP substrate 102. Spacing layer 302, side passivation layers 304, and top passivation layer 306 are also desirably formed of InP. Grating element 300 and waveguide 106 are desirably formed of InGaAsP material. The exemplary bandgap of grating element 300 is desirably 1.1 μm, while waveguide 106 desirably has a bandgap of 1.0 μm. Waveguide 106, in this exemplary embodiment, is 0.9 μm×0.9 μm. Grating element 300 are 3.0 μm×0.1 μm. Spacing layer 302 has a thickness of a 0.5 μm between waveguide 106 and grating element 300. This exemplary GDC desirably provides a periodic perturbation in the effective index of refraction along waveguide 106 of approximately 5×10−4.
The process begins with a substrate, step 400. Substrate 102, shown in
Next grating base layer 500 is grown, step 402. Metal organic chemical vapor deposition (MOCVD) is the preferred method for deposition of this sub-layer, but other epitaxial deposition techniques may also be employed, such as molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), and liquid phase epitaxy (LPE). This layer is the basis of the grating element and desirably is composed of InGaAsP with an index of refraction higher than that of the substrate.
Selective area growth (SAG) may be used to grow a layer of varying thickness. Therefore, the thickness of grating base layer 500 can easily be modified through the use of SAG to help provide apodization of the grating device, which is described below with reference to
To perform SAG of grating base layer 500 a patterned growth-retarding layer is formed on the top surface of the substrate before growing grating base layer 500. Materials which retard growth of III/V materials, such as SiN or SiO2, make up the growth-retarding layer. The growth-retarding layer may be formed and patterned using any standard techniques known in the semiconductor industry. In the areas covered by the growth retarding layer growth is substantially inhibited and material which falls upon the growth retarding layer diffuses laterally across the surface. Near the growth-retarding regions the growth rate is enhanced owing to gas phase diffusion and surface diffusion of the reactants in the MOCVD, or other epitaxial, reactor away from growth-retarding regions. After grating base layer 500 is grown the growth retard areas may be removed. Thickness variations of 5:1 or greater are attainable in this manner.
The grating base layer is defined and etched to form the grating element in step 404. A resist layer is applied on top of grating base layer 500 and patterned, as shown in
The grating base layer is then etched, and the resist is removed. This etch procedure preferably uses a wet etch technique, but a dry etch or combination may be employed. A wet etch may provide less abrupt edges. Grating base layer 500 may be etched to form grating element 600 with an undulating surface, such as shown in
In an alternative embodiment, the forming, defining, and etching grating base layer 500, steps 402 and 404, may be performed by an SAG technique. Patterned growth-retarding regions may be formed to induce grating base layer 500 to be formed with a desirably undulating surface so that no etching is needed. A grating base layer formed in this manner may additionally be designed for apodization at the same time.
Next, grating top layer 602 (shown in phantom), which includes the spacing layer, is formed over grating element 600 (shown in
It may also be desirable to use SAG techniques, described above with respect to step 402, on the grating top layer to produce a variation in the thickness of the spacing layer to assist in apodization of the exemplary device. Exemplary GDC's including variable thickness spacing layer 905 are shown in
Once grating layer 104 is completed at step 406, waveguide layer 700 is formed over grating layer 104, preferably by an epitaxial growth technique, step 408.
The growth of a quantum well structure in the waveguide layer may provide a waveguide that is more sensitive to electrical, temperature, and/or pressure variations. It is contemplated that this increased sensitivity may be exploited to increase the range, and possibly speed, of dynamic tunability which may be achieved in exemplary tunable GDC's, such as those shown in
Waveguide layer 700 is next defined and etched to form waveguide 106, step 410. Wet or dry etch techniques may be used. Alternatively, it may be desirable to first dry etch a structure slightly larger than the desired waveguide dimensions and then use a selective InGaAsP undercut etch to achieve the desired size for waveguide 106. This two part etching technique may provide a highly controllable method to form waveguide 106. As noted above, the cross-sectional shape and size of waveguide 106 may be important to minimize polarization modal dispersion.
Next passivation layer 108 is formed over waveguide 106, step 412. This step of the fabrication process is illustrated in
Following step 412,
Two alternative procedures, also shown in
The electrical contacts formed on the electrically tunable exemplary GDC's of
As shown in
These contacts may be formed using standard semiconductor fabrication techniques. The dynamically tunable exemplary dispersion compensating grating device is then completed by forming the waveguide I/O surface, step 414 as described above.
The final alternative exemplary procedure included in
Next, heatsink 1302 is thermally coupled to substrate 102, step 420. The heatsink may also be coupled the back side of the exemplary temperature tunable GDC as shown in
This alternative exemplary procedure also ends with the formation of waveguide I/O surface 107. It is noted that step 420 may be performed during packaging of the exemplary temperature tunable GDC, after step 414.
The process begins with a substrate, step 1800. Substrate 102, shown in
First, the waveguide layer is formed over substrate 102, preferably by an epitaxial growth technique, step 1802. The waveguide layer is preferably formed of InGaAsP having an index of refraction higher than the refractive index of the substrate. The waveguide layer may consist of a single layer of bulk material, a dual layer of with one n-doped portion and one p-doped portion or a plurality of layers forming a single or multiple quantum well structure.
The waveguide layer is next defined and etched to form waveguide 106, step 1804. Wet or dry etch techniques may be used. Alternatively, as described above, it may be desirable to first dry etch a structure slightly larger than the desired waveguide dimensions and then use a selective InGaAsP undercut etch to achieve the desired size for waveguide 106. This two part etching technique may provide a highly controllable method to form waveguide 106.
Next, spacing layer 1900 which includes the grating base layer is grown, step 1806. MOCVD is the preferred method for deposition of this sub-layer, but other epitaxial deposition techniques may also be employed, such as MBE, CBE, and LPE. This layer is the base of the grating element and desirably is composed of InP with an index of refraction similar to that of the substrate. The top surface of spacing layer 1900 is planarized, step 1808, to prepare for etching the grating base layer.
The grating base layer is defined and etched to form grating element 2000 in step 1810. A resist layer is applied on top of the grating base layer and patterned, as shown in
The grating base layer is then etched, and the resist is removed forming grating element 2000, shown in
Next, the grating top layer (not shown in
Next passivation layer 108 is formed over optical grating 104, step 1814. Preferably, passivation layer 108 is formed using the same method and material as spacing layer 1900. Passivation layer 108 desirably has a refractive index lower than that of waveguide 106, preferably similar to that of substrate 102 and spacing layer 1900, to act as a cladding layer and ensure light confinement within waveguide 106.
Exemplary GDC 2100, shown in
It is desirable that the interaction between the grating element and the optical signal travelling within the waveguide of an exemplary GDC decrease near the ends of the exemplary GDC. This apodization may be used to provide improved dispersion characteristics in the exemplary GDC. An exemplary apodization profile that may desirably be used is a super-Gaussian profile. Using this profile the interaction between the grating element and the optical signal is approximately zero at the front and back ends of the GDC waveguide. This interaction increases as a Gaussian toward the middle of the waveguide, but rather than peaking in a single position at the center, the super-Gaussian shape has an extended flat peak throughout the central portion of the waveguide
The apodization method illustrated in
The three exemplary apodization techniques described above may be used individually or may be combined. All three techniques may be use with grating elements that have a constant longitudinal spacing or with grating element that have a variable longitudinal spacing as illustrated in
The period of optical grating element 104 experienced by optical signal 109 depends on the effective index of refraction, neff, of the GDC. The value of neff is perturbed slightly by the indices of refraction of optical grating element 104 and passivation layer 108, but is largely determined by the index of refraction of quantum well waveguide 1702. This allows wavelength dependent chirp of the dispersion curve of exemplary GDC 1700 to be controlled by varying the thickness of quantum well waveguide 1702. It is contemplated that the period optical grating element 104 may be constant, or may be chirped. It is also contemplated that quantum well waveguide 1702 may be curved in the same way as curved waveguide 1102 in
The quantum wells and barriers of quantum well waveguide 1702 are preferably InxGa(1-x)ASyP(1-y) materials as well as InxAlyGa(1-x)As (1-y) and Inx)As materials. Specific selections of x and y are dependent on the desired bandgap and strain, if any, desired. These sub-layers may also be formed by other permutations of alloys formed from these elements.
Static GDC's are passive optical devices and, therefore, have no power consumption and require very minimal maintenance once they are installed. Dynamically tunable GDC's, however, may offer a number of advantages over static GDC's. Dynamically tunable GDC's may be designed to provide dynamic tuning of the center wavelength, or they may be designed to provide dynamic tuning of the dispersion characteristics of the device across the wavelength band desired to be compensated, or both.
This means that a dynamically tunable GDC may provide superior performance by tracking shifts in the optical wavelength due to age or environmental conditions. A common use for dispersion compensation elements is to compensate for the broadening of signal pulse due to chromatic dispersion from transmission over long-haul optical fibers, or from non-linear optical media in various optical devices. The shape, and magnitude, of the chromatic dispersion curve is dependent on a number of parameters, such as the length of the optical fiber, specific to each application. It is impractical to produce a static GDC for each individual use. One solution may be to produce standard sized static GDC's, generally with linear dispersion curves, and selected the closest one. With a dynamically tunable GDC, the dispersion characteristics may also be tuned during installation, or operation, to optimize performance in the specific application the dynamically tunable GDC is used in. Also, changes in the system configuration, such as adding a kilometer of optical fiber or a new semiconductor optical amplifier (SOA), may be compensated by changing the tuning of the dynamically tunable GDC.
Quantum well structures, including variable thickness quantum well structures such as those of quantum well waveguide 1702 of
The longitudinal variation in current density may also be manipulated by varying the thickness, and in turn the voltage of distribution, of resistive contact layer 1202. It is noted that resistive contact layer 1202 may also be omitted and the longitudinal spread of the current density may occur substantially in passivation layer 108. It is also noted that the choice to couple two electrodes to the top side of the exemplary current tunable GDC is merely illustrative. A greater number of electrodes may allow more sensitive control of the dispersion curve. Alternatively, it may be desirable to include only one electrode and to have the longitudinal current density distribution determined by the linear resistivity of resistive contact layer 1202 or the other layers of the exemplary current tunable GDC. Although, waveguide 106 may be a single layer bulk semiconductor material, it is contemplated that it may be desirable for waveguide 106 to be a dual layer material formed with a p-doped layer and an n-doped layer operating like a forward-biased diode junction, or a quantum well structure.
A longitudinally uniform change in temperature along the waveguide may have the effect of shifting the center wavelength of the GDC.
Heating element 1300 may be a thin film of deposited metal, which may only raise the temperature of the GDC above the ambient temperature through resistive heating, or it may be one or more thermoelectric coolers capable of raising and/or lowering the temperature as desired. In this relatively simple embodiment, it may be desirable to change to overall temperature of the GDC significantly from the ambient temperature to lower effects due to changes in the ambient temperature.
Finer control of the temperature gradient profile may be attained by the addition of further heating or cooling elements. Temperature sensors 1310, as shown in
The alternative exemplary temperature controlled GDC embodiment shown in
Alternatively, waveguide 106 may be formed of a piezoelectric material, or only a sub-layer of piezoelectric may be formed within the passivation layer to craft the pressure due to expansion, or contraction.
It is also contemplated that electrodes 1400 may be configured to provide substantially vertical, rather than substantially horizontal, pressure on waveguide 106. This alternative method of pressure tuning neff may be particularly effective for waveguides with quantum well structures. A small change in the thickness of the quantum well layers may significantly change the bandgap of such a structure. Shifting the bandgap effects both the absorption spectrum of the structure, but based on the Kramers-Kronig relationship the wavelength dependent index of refraction as well.
One exemplary use of a GDC is shown in
The broadened optical signal is coupled from fiber optics system 1500 into circulator 1504 via optical coupler 1502. Circulator 1504 routes the broadened optical signal through I/O coupler 1506 and into GDC 100. The optical signal is substantially recompressed in GDC 100 and reflected back into I/O coupler 1506. Circulator 1504 routes the recompressed optical signal into detector 1510 through optical coupler 1508. I/O coupler 1506 and optical couplers 1502 and 1508 may include optical fibers, planar waveguides, and/or lens systems.
Circulator 1504, I/O coupler 1506, and GDC 100 may be formed together as a monolithic dispersion compensation optical chip. Additionally, optical coupler 1508 and detector 1510 may be added to the optical chip to form a monolithic, dispersion compensated, optical signal detector.
The range over which an optical signal can be transmitted in such a system is limited by losses as well as pulse broadening within long-haul optical fibers. One solution is to detect the signal before it becomes undetectable and then retransmit the signal. This slows the overall transmission speed of the system and may introduce errors.
The broadened optical signal is coupled from long-haul optical fiber 1600 into circulator 1504 via optical coupler 1502. As in the detector system of
As an alternative embodiment, it may be desirable to place SOA 1602 before circulator 1504. This embodiment may allow chromatic dispersion which may be introduced by SOA 1602 to be compensated by GDC 100, as well as chromatic dispersion from long-haul optical fiber 1600. If the pulse broadening is too great, SOA 1600 not be able to operate efficiently in this configuration.
Another alternative embodiment of the exemplary extended range fiber optic communication system of
Transmissive planar waveguide optical component 2201 may be a passive optical element, such as a circulator, or may be an active optical element, such as a variable optical attenuation (VOA), an electro-absorption modulator (EAM), or an SOA, as shown in
The active optical element 2201 of exemplary monolithic optical chip shown in
Performance of quantum well structures in both GDC's and active optical elements, such as SOA's and VOA's, may be greatly affected by the thickness of the sub-layers of the quantum well structure. SAG may be used to produce a quantum well structure in active waveguide layer 2202 that has a different thickness than in GDC waveguide 2206. By producing different thicknesses for active waveguide layer 2202 and GDC waveguide 2206, each portion of the waveguide may be optimized. The waveguide portion between active waveguide layer 2202 and GDC waveguide 2206 may be gradually changed to allow adiabatic expansion and contraction of the optical signal modes, reducing any possible loss.
Although the embodiments of the invention described above have been mostly in terms of GDC's formed in III/V materials and with the optical grating formed beneath the waveguide, it is contemplated that similar concepts may be practiced with other dielectric materials, or may be practiced with the optical grating formed on top of the waveguide. Also, it will be understood to one skilled in the art that a number of other modifications exist which do not deviate from the scope of the present invention as defined by the appended claims.
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|U.S. Classification||385/37, 385/123|
|International Classification||G02B6/12, G02B6/34|
|Cooperative Classification||G02B6/12007, G02B2006/12107, G02B6/29398, G02B6/29395, G02B6/29325, G02B6/29394|
|European Classification||G02B6/12M, G02B6/293W10, G02B6/293W8C|
|Jun 11, 2002||AS||Assignment|
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